Due to the increasing demand for improved aesthetics, ceramic materials have been used more widely in various dental applications such as veneers, inlays, onlays, crowns, bridges (fixed dental restorations), and other dental restorations. In addition to increased translucency, ceramics possess good biocompatibility, and, in general, exhibit good abrasive resistivity, which makes ceramics attractive as dental materials. However, when ceramics were first introduced into dentistry, the relatively low mechanical strength of the ceramic materials was a common problem.
Ceramics and glasses are brittle in that those materials display a high compressive strength, but also display a low tensile strength and may fracture under very low strain (0.1%, 0.2%). In this regard, dental ceramics are commonly viewed as having a number of disadvantages when used as restorative materials (mostly due to their inability to withstand functional forces that are present in the oral cavity). Consequently, dental ceramics were not initially utilized in the premolar and molar areas. Further material development has enabled the use of ceramics in posterior restorations, as well as in structures over dental implants. Nevertheless, all dental ceramics still generally display low fracture toughness when compared with other dental materials such as metal-ceramic or specialty solo metals. Indeed, zirconia-based ceramics have been considered the strongest of all ceramic restorations available in the market. However, it was previously reported that the catastrophic failure rate within the zirconia core ceramic was around 7% for single crowns after 2 years, and 1% to 8% for fixed dental prostheses after 2-5 years. One major cause of that catastrophic failure was occlusal overload due to bruxism, which causes cracks at the cementation surface. Those cracks then propagate towards the surface, and eventually cause the entire restoration to fracture. In fact, in general, the survival rate for metal-ceramic restorations was ultimately found to be significantly higher than zirconia-based or any other ceramic restorations.
In addition to the problems inherent in a number of dental materials, in recent years, it has also been discovered that human dental structures are in fact structures with graded properties. The enamel of the tooth has an elastic modulus of about 80 GPa and hardness of about 4 GPa, while the dentin of the tooth has an elastic modulus of about 20 GPa and a hardness of about 1 GPa. Furthermore, even within the enamel itself, there exists a gradual change of mechanical properties from the inner region to the outer surface, which is largely influenced by a change of microstructure. It has also been found that upon loading, more energy is dissipated through the inner enamel, which undergoes viscoelastic deformation over a relatively large area, and thus the tooth may accommodate higher levels of strain. By comparison, most ceramic dental restoration parts have relatively homogenous mechanical properties throughout the structure. Therefore, the cores of the dental restorations often have significantly higher stiffness than that of the human dentin, making them less capable of absorbing strain and other forces applied to it.
Despite the recognition of graded properties in dental structures, such as enamel, it is almost impossible to create a dental part with a graded elastic modulus with currently known processes. The currently known sintering, casting, or milling processes are all incapable of producing controlled inhomogeneity, and machining processes can only produce external features. Some previous works have shown that the use of dental restorations with a non-uniform elastic modulus could significantly increase the mechanical performance of the zirconia parts. In particular, in one series of works, it was observed that fabricated graded structures could be produced using silica-alumina with a low elastic modulus and using zirconia with a high elastic modulus. By infiltrating glass into zirconia plates at both ends of the part, glass-ceramic-glass graded structures were created with relatively soft skins and stiff cores. Those results also showed a significant increase (20%-50%) in the fracture loads of the infiltrated material. Nevertheless, the ability to mimic the structure of a natural tooth and produce a graded elastic modulus was still limited in those studies.